Hi,
I have installed on my xp sp2 pro the avast home edition 4.7.
Every time that I startup my pc I see that avast run a primary scan for 30-40 seconds and in this time my pc is very slow. After the initial scan the pc return in normal condition.
Then, how I can disable the startup scan of avast?
Startup is a time when many viruses are loaded so it is advisable it runs early to prevent viruses loading into memory, etc.
What is your sensitivity level for Standard Shield ?
Normal is the default and provides a compromise between protection and performance.
Can you give us some idea of the performance of your system, CPU, RAM, etc. and what other security programs you might also have loading/running on startup ?
Ensure that only essential programs load on startup, media players are a big culprit and don’t need to load at startup only when a media file is run.
same problem for me… lots of files (428 !) are tested every time i boot, and my pc is very slow (and freezes a few seconds) for 1 min after windows gives me the hand… :-\
Try Startup Delayer.
It’s a freeware that controls the windows startup (Windows 98\Me\2k\XP) that does not follow a strict order.
You can set some applications to start very after the boot (logon) itself. 
Since avast! version 4.7.807 the Standard Shield has a feature called do not scan system libraries (by default on) that could dramatically decrease the number of files scanned during computer boot if you disable it.
Same questions as above then.
My startup scanned total is usually around 100 with standard shield on Normal and startup programs and services parred to an absolute minimum.